From Joomla! Documentation

User registrations are enabled in a default Joomla! installation. However there might be cases where you use extensions as a replacement and wish to turn off the default Joomla user management. To do this you must perform the following steps:

Joomla 1.5

User Registration

If you do not want visitors to your website to be able to register themselves as your Joomla! website users, do this: